Tea Tree (Melaleuca alternifolia)

The history of tea tree oil or Melaleuca alternifolia is extensive. As a native Australian plant, it is believed to have been widely used by the Bundjalung Aborigines of northern New South Wales for its medicinal properties (Hammer, Carson, & Riley, 1999). With a limited distribution and its affinity for swampy conditions, this plant is difficult to harvest in its natural environment. As a result, Australia is now home to many large plantations with the ability to produce adequate quantities of this useful oil. Plants are sustainably harvested 14-16 months after planting (Carson, 1998) and directly after flowering for the optimum antimicrobial effects (Burt, 2004). The plant’s volatile oil is extracted from the leaves and terminal branches with a yield of 1-2% of the dry weight (Carson, 1998).

Due to its antimicrobial, anti-inflammatory, and analgesic effects, tea tree oil is currently used as a topical agent in the treatment of multiple conditions: acne, tinea, insect bites, and burns. It also serves as a popular additive to many cosmetic and pharmaceutical products available worldwide (Hammer, Carson, & Riley, 1999).  Analysis has determined that terpinen-4-ol is the principal active ingredient which usually constitutes 40% of the oil (Cox, Mann, & Markhan, 2001). Through years of evaluation, it has been determined that the oil is comprised of approximately 100 different components with 14 labeled as major constituents. Each sample contains differing levels, so in an effort for consistency, an international standard was determined for these 14 components by the International Standardization Organization which stipulates a range for each (Carson, 1998).

The particular action of this oil on bacteria is unknown. It has been speculated that its ability to dissolve in lipids, in turn, affects the integrity of the cell membrane in some way (Carson, 1998). Healthy cell membranes provide a barrier to toxic substances and to the passage of small ions. The cell controls the entry and exit of different compounds, and this role of the membrane is integral to many cell functions. Detrimental effects on membrane structure ultimately lead to cell death (Cox et al, 2000). According to a study conducted by Cox et al., tea tree oil disrupted the cell membrane resulting in the interruption of several vital cell functions. At varying concentrations, tea tree oil affected cell respiration, cell viability, and induced leakage of potassium ions from three organisms, E. coli, S. aureus, C. albicans. An additional study added support to the theory of gross membrane damage by treating S. aureus with tea tree and subsequently exposing the bacterial cells to sodium chloride. Healthy cells of S. aureus would naturally prevent an influx of toxic substances such as salt; however, the researchers observed that these treated cells were unable to do so. The compromised cell membranes ultimately caused the cells to die (Carson, 1998).

Currently the extent of tea tree’s antimicrobial properties has been under limited study by means of in vitro. The standardized test methods of MIC, or minimum inhibitory concentration, are normally used to assess oil activity. These tests determine the smallest amount of oil needed to inhibit the growth of a test organism (Brock, Madigan, Martinko, & Parker, 1994). Typically several industry-standard pathogens are subjected to MIC testing, and results recorded. As representatives of each microbial family, Cox et al. uses E. coli, S. aureus, and C. albicans to determine the concentration of oil necessary to inhibit microbial growth and to achieve complete microbial kill. Findings concluded that all organisms were susceptible to the oil’s activity at a concentration of .5% or less (Cox et al, 2000) which easily falls into the usual commercial concentration of 2-5% that can be found in tea tree oil products (McCaleb 1996). It has been determined, however, that certain bacteria exhibit more tolerance and require higher concentrations. It has been speculated that this phenomenon can be attributed to differences in the outer cell membrane in different organisms, such as Pseudomonas aeruginosa, which directly affects tea tree’s ability to damage the membrane in its usual manner (Halcon & Milkus, 2004).

Tea tree oil has shown great promise with other organisms, however, that are commonly known to have a propensity for resistance. Edwards-Jones, Buck, Shawcross, Dawson & Dunn (2004) have studied methicilllin-resistant S. aureus that has become the epidemic of burn victims in hospitals around the country. Seventy-five percent of deaths in burn patients are caused by wound complications due to advanced infections of the bloodstream, and this strain is targeted as a big contributor. While this organism is now antibiotic resistant, other alternative options are being explored. Through studies involving MIC, disc diffusion, and wound dressings, Edwards-Jones, et al. (2004) have determined that tea tree when used as a single oil is the most effective at inhibiting this difficult organism.

Up to this point, clinical research is extremely limited. Those completed studies have demonstrated efficacy but are not a comprehensive representation, as most are uncontrolled case studies using a small number of patients (Halcon & Milkus 2004).  Further work needs to be done in order to determine if tea tree oil will be useful in vivo; however, as suggested from previous in vitro testing, this essential oil seems to exhibit potential. It has exhibited antibacterial and antifungal properties which includes destroying particular strains resistant to antibiotics, but researchers speculate that certain gram negative organisms could be immune to this action.